Gas delivery masks are widely used for medical purposes to deliver oxygen and other breathable gases to a patient. Typically, a conventional mask is configured to substantially cover the nose and mouth region of a patient. A conventional mask includes an inlet for connection with a source of pressurized gas. The mask may also include a baffle, diffuser or other structure to generate a gas plume within the interior of the mask or otherwise control the gas flow properties.
Gas delivery masks are described in the following references:    U.S. Pat. No. 7,934,501    U.S. Pat. No. 8,042,540    U.S. Pat. No. 9,199,052    US 2013/0186406    USA 2017/0095633
Gas delivery masks for medical use face competing requirements. On the one hand, the mask must be able to deliver oxygen or other gas at a relatively high concentration to the patient. Normally, this requires a relatively high flow rate of gas. However, on the other hand, the mask should be comfortable to the user, which generally is improved by a lower rate of gas flow; as well, a lower gas flow rate is more economical to the hospital. As such, it is desirable to improve the efficiency of gas delivery in a mask in order to improve patient comfort and reduce the gas flow rate required to deliver a given gas concentration to the patient. Reduction in gas flow rate also tends to reduce the noise generated by the device.